Carboxylic acids are valuable chemicals that are employed in many applications in industry. For example, acetic acid has a wide range of uses, including in the chemical industry to produce cellulose acetate, rayon acetate, acetic anhydride and plastics and in the food industry as a preservative. Acetic acid is produced both synthetically and by bacterial fermentation. Most of the acetic acid produced for the chemical industry is made by methanol carbonylation, whereby methanol and carbon monoxide are reacted to produce acetic acid. Acetic acid that is used as a food additive is produced by the biological route, as many nations' food purity laws stipulate that vinegar used in foods must be of biological origin. Other carboxylic acids of industrial importance include formic acid and propionic acid. Formic acid can react to form esters and is used as a preservative in animal feeds while propionic acid is the flavorant in Swiss cheese.
Recovering acetic acid that is produced as a byproduct from lignocellulosic conversion processes has received much attention in recent years. Agricultural wastes are of particular interest as they are inexpensive, and are often burned or landfilled. There is an enormous untapped potential for their use not only as a source of fermentable sugar to produce fuels such as ethanol or butanol, but also as a source of byproducts, such as acetic acid
In the production of fermentable sugar from lignocellulosic feedstocks, the acetic acid arises from the hydrolysis of acetyl groups present on the hemicellulose and lignin components of the feedstock. For instance, the acetic acid may originate from an acid pretreatment, which is conducted to hydrolyze the hemicellulose component of the feedstock, but with limited hydrolysis of the cellulose. The cellulose is then hydrolyzed with cellulase enzymes and the glucose so produced is fermented to ethanol, butanol or other fermentation products. Other known methods for producing sugar hydrolyzate streams that also contain acetic acid or acetate salts include alkali pretreatment conducted under conditions that result in hemicellulose hydrolysis, followed by enzymatic hydrolysis of cellulose with cellulase enzymes or complete acid hydrolysis conducted in a single step under harsher conditions so that both the hemicellulose and cellulose present in the feedstock are hydrolyzed. Acetic acid can also be produced as a byproduct in other industries that utilize lignocellulosic materials as feedstocks, including during furfural production and in the pulp and paper industry.
Formic acid is also a byproduct produced during the pretreatment of lignocellulosic feedstocks, specifically by sugar and lignin degradation that occurs during such processes. Formic acid is also produced as a byproduct during furfural production from lignocellulosic feedstocks, along with acetic acid.
Whether or not the recovery of carboxylic acids from industrial process streams is feasible depends on the cost of the recovery, the ability to remove impurities and the ability to concentrate it to a sufficiently high concentration (e.g., in the case of acetic acid as glacial acetic acid). Streams derived from lignocellulosic feedstocks pose particular problems for successful recovery of carboxylic acids due to their multi-component nature and because the concentration of carboxylic acids in such streams is typically low.
Liquid-liquid extraction is a known technique for recovering carboxylic acids. This method, also known as solvent extraction, extracts carboxylic acids with a solvent or mixture of solvents to produce an extract containing the acid and the extracting solvent and typically some of the water in the process stream. The extract may be distilled to recover the extracting solvent for reuse in the process and to obtain a concentrated acid solution free of the solvent. Such extractions may involve the use of organic bases such as alkylamines and phosphine oxides. (See for example Ricker, N. L., Pittman, E. F., and King, C. J., J. Separ. Proc. Technol., 1980, 1(2):23-30).
However, the recovery of carboxylic acids by liquid-liquid extraction at the low concentrations found in process streams resulting from lignocellulosic conversion process, e.g., less than about 2% (w/w), requires significant amounts of the organic solvent in order for the extraction to be effective. This is a major disadvantage as such solvents are costly. Moreover, the solvent often has a high affinity for lignin and other high molecular weight compounds that are present in many of the streams produced during the conversion process. These compounds can accumulate in the solvent and render it less effective. Furthermore, the use of agitation to increase the rate of liquid-liquid extraction often leads to the formation of emulsions of droplets of the aqueous phase within the organic phase. The separation of the emulsified phases can be difficult. Accordingly, liquid-liquid extraction is not preferred for directly recovering acetic acid from streams containing these components.
British Patent No. 1,407,523 discloses a method of recovering acetic acid by extractive rectification. According to the method, a crude acid mixture containing acetic acid is fed into the lower half of a first rectification column either in liquid form or the form of a vapour. The extractant, 1,2-dimorpholylethane, is fed as a liquid in the upper third of the column. The sump product of the first column, which consists of an anhydrous mixture of acetic acid and extractant, is fed continuously to the lower half of the second rectification column. Acetic acid, which is free from water and extractant is taken off as distillate, while a product, consisting essentially of the extractant is obtained as a sump product. A similar process is disclosed by U.S. Pat. No. 3,951,755 (Sartorius et al.) using N-methyl acetamide as the extractant for acetic acid. CN101306989 discloses using a thiocyanate, acetate or nitrate salt in combination with an organic solvent for separating water and acetic acid by extractive distillation. Moreover, Lei et al. (Separation and Purification Technology, 2004, 36:131-138) discloses a “complex extractive distillation” for separating acetic acid and water using tributylamine as the separating agent. However, distillation is a very capital intensive process. Because of this, it is generally conceded as not being worthwhile for concentrating dilute aqueous acetic acid having less than about 30 weight percent acetic acid.
Another method of recovering acetic acid from an aqueous stream involves evaporating the acetic acid and water and then condensing the vapours thus formed, followed by extracting the acetic acid from the condensate by liquid-liquid extraction. Such processes are disclosed by U.S. Pat. No. 4,401,514 (Kanzler et al.) and U.S. Pat. No. 4,102,705 (Pfeiffer et al.). However, condensation and cooling of the vapour requires additional equipment and a large amount of energy, which increases the complexity and cost of the process.
The recovery of acetate salts using evaporation has been disclosed. This involves evaporating acetic acid from solution and contacting the vapourized acetic acid produced in the evaporator with alkali, thereby producing an acetate salt. For example, U.S. Pat. No. 1,314,765 discloses recovering acetic acid from the vapours of vegetable extracts undergoing evaporation in multiple evaporation units. The process involves intimately contacting alkali, such as lime, in the form of a spray, with vapours passing from one unit to another, thereby producing the acetate salt.
U.S. Pat. No. 114,517 discloses a process whereby acetate salt of lime is recovered from acetic acid vapours by contacting the vapours with lime that is placed on trays in a cylindrical vessel. Moreover, U.S. Pat. No. 1,052,446 discloses a process of making acetate of lime that involves contacting vapours containing acetic acid with a hot calcium carbonate solution.
Likewise, U.S. Pat. No. 4,898,644 (Van Horn) discloses a process for recovering an acetate salt as a byproduct produced during the production of furfural. The process involves steam stripping organic acids, including acetic and/or formic acid from an aqueous solution containing same, and contacting the vapourized acetic acid with sodium hydroxide to form sodium acetate. Prior to removing the acetic acid, furfural may be removed from the feed stream in a furfural stripper.
However, a disadvantage of the processes of U.S. Pat. Nos. 4,898,644, 1,314,765, 114,517 and 1,052,446 is that a further step of acidification would be necessary to further purify and recover acetic acid from the solution containing the sodium acetate or calcium acetate. Prior to extraction with a solvent, acidification is necessary so that sodium acetate or calcium acetate is in the non-dissociated form, (i.e., so that it is present predominantly as the acetic acid species, rather than the acetate salt species) and this is typically carried out by using sulfuric acid, which is costly and creates sulfate salts that must be processed. Furthermore, this purification step necessitates a separate liquid-liquid extraction to recover the acetic acid. The increased chemical usage by the acidification and the requirements for additional equipment increase the cost and complexity of the process, which in turn has a negative impact on the economics of the process.
As noted previously, it is known to recover acetic acid as a byproduct during the production of furfural. Furfural is produced from the decomposition of xylose that results from the hydrolysis of the hemicellulose component of lignocellulosic feedstocks, such as wood chips. During such production processes, the raw material is fed into a reactor operating at high temperatures by the introduction of steam to produce furfural, as well as the byproducts, methanol, formic acid and acetic acid. Vapour flowing from the reactor contains water, furfural, formic acid and acetic acid and it is known to separate these acids from one another from this vapour stream and subsequently purify them.
For example, U.S. Pat. No. 4,088,660 (Puurunen) discloses such a process for producing furfural and recovering acetic acid as a byproduct. According to this process, the vapour stream produced from the reactor, containing the furfural, methanol, acetic acid and formic acid, is contacted with furfural in a gas washer and, subsequently, in an absorption tower. The furfural, which is recycled from the process, serves to absorb the acetic acid and part of the water from the vapour, thus producing an aqueous solution containing the organic acids and furfural. This aqueous solution is then dehydrated and subjected to distillation in order to separate the volatile organic acids from the furfural.
However, a drawback of the above process of Puurunen (supra) is that the solubility of furfural in water is 8.3% (83 g/L) and the solubility of water in furfural is about 5%, depending on the temperature. These mutual solubilities are too high for furfural to be an effective extractant of acetic acid from water. That is, the loss of furfural in the water phase and the need to remove water from the furfural phase would add significant cost to the operation. In addition, the extraction of acetic acid by furfural is very weak. The concentration of acetic acid in furfural is less than that in water in an acetic acid-furfural-water extraction system at 35° C. (E. L. Heric and R. M. Rutledge, (1960), Journal of Chemical Engineering Data 5(3): 272-274).
Zeitsch (The Chemistry and Technology of Furfural and its Many Byproducts (2000), ACS Sugar Series, Vol. 13, Elsevier, Köln, Germany, p. 111-113) discloses the use of triethylamine vapour to extract acetic acid vapour and purify it from an aqueous solution. Triethylamine has a boiling point of 89° C. However, triethylamine reacts with acetic acid to form a complex with a high boiling point (165° C.), which complex can be separated from water by distillation. The complex can then be split by reacting it with ethanol at elevated temperature in the presence of an ion exchange resin which produces ethyl acetate, from which acetic acid can be produced. However, since the process is complicated and requires many steps it is impractical for use on an industrial scale.
At present, none of the prior art addresses operating an efficient and economical process for recovering volatile carboxylic acids, such as at the low concentrations found in many industrial process streams, including streams obtained from lignocellulosic conversion processes. The development of such a recovery process remains a critical requirement for the utilization of carboxylic acids as byproducts of economic significance.